An AC system having, for example, a voltage source (1), an inductance (2) and a load (3) has connected thereto an apparatus for controlling the phase and amplitude of the voltage of the system. The instantaneous current and voltage in the AC electrical system are measured (410, 440) and power measurements derived (430) therefrom. The power measurements may be instantaneous real and imaginary power, instantaneous active and reactive power, or active and reactive power averaged over less than one cycle. The power measurements are then used to derive (421, 422) phase related control values (Iq, Id), on the basis of which control signals are generated to a converter (42). The action of the converter (42) in accordance with the control signals thus controls the amplitude and phase of the voltage. By use of power measurements, it is not necessary to measure the phase of the voltage of the AC system.